Epitaxially deposited semiconductor layers, preferably of SiGe or Si, involving high and sharply delimited dopings, are increasingly used in the production of high frequency devices such as for example 1-heterobipolar transistors (HBT) and in CMOS circuits. Those highly doped layers involve the problem of diffusing out during subsequent process steps at elevated temperatures within the technological procedures involved and thus degradation of the electronic properties of those layers. In order to minimize those diffusion processes the temperatures have to be reduced and additional diffusion-inhibiting materials have to be employed.
WO 98/26457 describes how the diffusion of boron in SiGe is significantly suppressed by the use of an additional, electrically non-active material, preferably an element from the 4th main group, in particular carbon, in a concentration of between 1018 cm−3 and 1021 cm−3. The production of those epitaxial diffusion-inhibiting layers, preferably of SiGeC or SiC, is effected by molecular beam epitaxy (MBE) and primarily with chemical gaseous phase deposition (CVD) processes. As described in T. I. Kamins, D. J. Meyer, Appl. Phys. Lett., 59, (1991) 178; W. B. de Boer, D. J. Meyer, Appl. Phys. Lett. 58, (1991) 1286 and B. S. Meyerson, Appl. Phys. Lett. 48 (1986) 797, single-wafer and ultra-high-vacuum (UHV) batch reactors are used in conjunction with CVD processes. In the case of the single-wafer reactors, it can be detrimentally observed that those installations do not involve hot wall reactors, in other words in single-wafer reactors the wafers are very rapidly heated by means of radiant or induction heating, in which case neither the wafer nor the reactor goes into a condition of thermodynamic equilibrium.
Because of the necessary low deposition rates in the case of Si-, SiC-, SiGe- and SiGeC-low temperature epitaxy, only a low throughput rate is to be achieved with single-wafer reactors. For typical HBT stacks the throughput rate is for example about 5 wafers per hour. That is not economically viable for an industrial process.
The sole batch reactor which has been known hitherto is an UHV hot wall reactor which operates in the temperature range of between 400° C. and 800° C. and typically at 600° C. In those hot wall reactors the wafers are heated in small batches in a condition of thermodynamic equilibrium, whereby it is admittedly possible to achieve a substantially better level of temperature homogeneity, but the high expenditure linked to the UHV process has a disadvantageous effect on the throughput rate. Thus, all peripheral process times (for example pumping and flushing sequences, handling of the wafers and so forth) are substantially longer than for example with conventional low pressure (LPCVD) installations. Therefore, the throughput rate for typical HBT stacks in the case of that UHV batch reactor is in the range of the single-wafer reactors, that is to say about 5 wafers per hour. There is also the disadvantageous effect that, because of the UHV system, it is not possible to use very high temperatures (1000° C. and higher) and thus for example in the UHV batch reactor etching and baking processes can only be implemented at low temperatures. The low H2O/O2 residual content for low temperature epitaxy, which is achieved for UHV installations and which is represented as necessary, can also be achieved by suitable measures for non-UHV installations.
Low pressure (LP) batch reactors were hitherto not used for the production of diffusion-inhibiting semiconductor layers, in particular not those of SiGeC or SiC. The reason for this is on the one hand the fact that the low oxygen and moisture content required by the men skilled in the art for low temperature epitaxy in those installations was deemed not to be a feasible proposition. On the other hand, depletion effects occur in the gaseous phase in high temperature epitaxy in the batch reactors, and those effects result in inadequate homogeneity of the deposited layers on the substrates. Therefore single wafer reactors have gained general acceptance for high temperature epitaxy (T≧1000° C.). At low temperatures however kinetic effects dominate and transport-conditioned depletion is of subordinate significance in comparison with the influence of temperature homogeneity on layer homogeneity.